Central motor pathways in patients with mirror movements

Ipsilateral motor evoked potentials in a patient with unihemispheric cortical atrophy due to Rasmussen encephalitis

The role of the ipsilaterally descending motor pathways in the recovery mechanisms after unilateral hemispheric damage is still poorly understood. Motor output reorganization was investigated in a 56-year-old male patient with acquired unilateral hemispheric atrophy due to Rasmussen encephalitis. In particular, the ipsilateral corticospinal pathways were explored using focal transcranial magnetic stimulation. In the first dorsal interosseous and wrist extensors muscles, the median amplitudes of the ipsilateral motor evoked potentials induced by transcranial magnetic stimulation in the patient were higher than those of 10 age-matched healthy control subjects. In the biceps brachii muscle, the median amplitudes of the ipsilateral motor evoked potentials were the second largest in the patient compared to the controls. This study demonstrated a reinforcement of ipsilateral motor projections from the unaffected motor cortex to the hemiparetic hand in a subject with acquired unihemispheric cortical damage.

Predicting hand function after hemidisconnection

Hemidisconnections (i.e. hemispherectomies or hemispherotomies) invariably lead to contralateral hemiparesis. Many patients with a pre-existing hemiparesis, however, experience no deterioration in motor functions, and some can still grasp with their paretic hand after hemidisconnection. The scope of our study was to predict this phenomenon. Hypothesizing that preserved contralateral grasping ability after hemidisconnection can only occur in patients controlling their paretic hands via ipsilateral corticospinal projections already in the preoperative situation, we analysed the asymmetries of the brainstem (by manual magnetic resonance imaging volumetry) and of the structural connectivity of the corticospinal tracts within the brainstem (by magnetic resonance imaging diffusion tractography), assuming that marked hypoplasia or Wallerian degeneration on the lesioned side in patients who can grasp with their paretic hands indicate ipsilateral control. One hundred and two patients who underwent hemidisconnections between 0.8 and 36 years of age were included. Before the operation, contralateral hand function was normal in 3/102 patients, 47/102 patients showed hemiparetic grasping ability and 52/102 patients could not grasp with their paretic hands. After hemidisconnection, 20/102 patients showed a preserved grasping ability, and 5/102 patients began to grasp with their paretic hands only after the operation. All these 25 patients suffered from pre- or perinatal brain lesions. Thirty of 102 patients lost their grasping ability. This group included all seven patients with a post-neonatally acquired or progressive brain lesion who could grasp before the operation, and also all three patients with a preoperatively normal hand function. The remaining 52/102 patients were unable to grasp pre- and postoperatively. On magnetic resonance imaging, the patients with preserved grasping showed significantly more asymmetric brainstem volumes than the patients who lost their grasping ability. Similarly, these patients showed striking asymmetries in the structural connectivity of the corticospinal tracts. In summary, normal preoperative hand function and a post-neonatally acquired or progressive lesion predict a loss of grasping ability after hemidisconnection. A postoperatively preserved grasping ability is possible in hemiparetic patients with pre- or perinatal lesions, and this is highly likely when the brainstem is asymmetric and especially when the structural connectivity of the corticospinal tracts within the brainstem is asymmetric.

Interhemispheric control of unilateral movement

To perform strictly unilateral movements, the brain relies on a large cortical and subcortical network. This network enables healthy adults to perform complex unimanual motor tasks without the activation of contralateral muscles. However, mirror movements (involuntary movements in ipsilateral muscles that can accompany intended movement) can be seen in healthy individuals if a task is complex or fatiguing, in childhood, and with increasing age. Lateralization of movement depends on complex interhemispheric communication between cortical (i.e., dorsal premotor cortex, supplementary motor area) and subcortical (i.e., basal ganglia) areas, probably coursing through the corpus callosum (CC). Here, we will focus on transcallosal interhemispheric inhibition (IHI), which facilitates complex unilateral movements and appears to play an important role in handedness, pathological conditions such as Parkinson's disease, and stroke recovery.

Neural synchrony within the motor system: what have we learned so far?

Synchronization of neural activity is considered essential for information processing in the nervous system. Both local and inter-regional synchronization are omnipresent in different frequency regimes and relate to a variety of behavioral and cognitive functions. Over the years, many studies have sought to elucidate the question how alpha/mu, beta, and gamma synchronization contribute to motor control. Here, we review these studies with the purpose to delineate what they have added to our understanding of the neural control of movement. We highlight important findings regarding oscillations in primary motor cortex, synchronization between cortex and spinal cord, synchronization between cortical regions, as well as abnormal synchronization patterns in a selection of motor dysfunctions. The interpretation of synchronization patterns benefits from combining results of invasive and non-invasive recordings, different data analysis tools, and modeling work. Importantly, although synchronization is deemed to play a vital role, it is not the only mechanism for neural communication. Spike timing and rate coding act together during motor control and should therefore both be accounted for when interpreting movement-related activity.

Spectral changes of interhemispheric crosstalk during movement instabilities

Bimanual coordination requires the functional integration of the activity in various cortical, subcortical, spinal, and peripheral neural structures. We challenged this functional integration by destabilizing bimanual 5:8 tapping through an increase in movement tempo, while measuring brain and muscle activity using magnetoencephalography and electromyography. Movement instabilities were characterized by a drop in frequency locking. Time-frequency analysis revealed movement-related beta amplitude modulation in bilateral motor areas as well as movement-related corticospinal entrainment. Both of these synchronization patterns depended on movement tempo suggesting that the timescale needed for the upregulation and downregulation of beta synchrony in rhythmic tapping poses constraints on motor performance. Bilateral phase locking over movement cycles appeared to be mediated by beta-frequency oscillations and constrained by its phase dynamics. The timescale of beta synchrony thus seems to play a key role in achieving timed phase synchrony in the motor cortex and along the neural axis. Once event-related desynchronization-synchronization cycles cannot be build up properly, inhibition may become inadequate, resulting in a reduction of the stability of performance, which may eventually become unstable.

Hemispheric asymmetry of frequency-dependent suppression in the ipsilateral primary motor cortex during finger movement: a functional magnetic resonance imaging study

Electrophysiological studies have suggested that the activity of the primary motor cortex (M1) during ipsilateral hand movement reflects both the ipsilateral innervation and the transcallosal inhibitory control from its counterpart in the opposite hemisphere, and that their asymmetry might cause hand dominancy. To examine the asymmetry of the involvement of the ipsilateral motor cortex during a unimanual motor task under frequency stress, we conducted block-design functional magnetic resonance imaging with 22 normal right-handed subjects. The task involved visually cued unimanual opponent finger movement at various rates. The contralateral M1 showed symmetric frequency-dependent activation. The ipsilateral M1 showed task-related deactivation at low frequencies without laterality. As the frequency of the left-hand movement increased, the left M1 showed a gradual decrease in the deactivation. This data suggests a frequency-dependent increased involvement of the left M1 in ipsilateral hand control. By contrast, the right M1 showed more prominent deactivation as the frequency of the right-hand movement increased. This suggests that there is an increased transcallosal inhibition from the left M1 to the right M1, which overwhelms the right M1 activation during ipsilateral hand movement. These results demonstrate the dominance of the left M1 in both ipsilateral innervation and transcallosal inhibition in right-handed individuals.

Ipsilateral motor activation during unimanual and bimanual motor tasks

OBJECTIVE

To test for the presence and possible asymmetry of ipsilateral motor activation during unimanual and bimanual motor tasks.

METHODS

Twelve right-handed healthy subjects underwent motor evoked potential (MEP) measurement of one hand (target-hand) while the other hand (task-hand) performed different motor tasks. The target-hand was either at rest (first experiment) or performed a Perdue PegBoard task (second experiment). The task-hand was either at rest, performed a simultaneous pegboard task, or rotated a coin (second experiment).

RESULTS

In the first experiment, the motor task resulted in significant increase in MEP area in the target-hand, regardless which hand was the task-hand, with a greater increase when the left hand was the task-hand. In the second experiment, ipsilateral motor activation was not present for either hand, however, when the right hand was the task-hand, performance of continuous coin rotation by the right hand resulted in a significant decrease in the MEP area of the left hand.

CONCLUSIONS

Hemispheric asymmetry and task-dependence of ipsilateral motor cortex activation supports the postulate that motor activity may start bilaterally with subsequent interhemispheric inhibition. Furthermore, in right-handers, the left motor cortex is either more active in ipsilateral hand movements or exerts more effective inhibitory control over the right motor cortex than vice versa.

SIGNIFICANCE

We suggest that hemispheric asymmetry in ipsilateral motor control is a factor in determining motor dominance in right-handed individuals.

Ipsilateral corticospinal projections do not predict congenital mirror movements: a case report

Congenital mirror movements (CMMs) are involuntary, symmetric movements of one hand during the production of voluntary movements with the other. CMMs have been attributed to a range of physiological mechanisms, including excessive ipsilateral projections from each motor cortex to distal extremities. We examined this hypothesis with an individual showing pronounced CMMs. Mirror movements were characterized for a set of hand muscles during a simple contraction task. Transcranial magnetic stimulation (TMS) was then used to map the relative input to each muscle from both motor cortices. Contrary to our expectations, CMMs were most prominent for muscles with the strongest contralateral representation rather than in muscles that were activated by stimulation of either hemisphere. These findings support a bilateral control hypothesis whereby CMMs result from the recruitment of both motor cortices during intended unimanual movements. Consistent with this hypothesis, bilateral motor cortex activity was evident during intended unimanual movements in an fMRI study. To assess the level at which bilateral recruitment occurs, motor cortex excitability during imagined unimanual movements was assessed with TMS. Facilitory excitation was only observed in the contralateral motor cortex. Thus, the bilateral recruitment of the hemispheres for unilateral actions in individuals with CMMs appears to occur during movement execution rather than motor planning.

Electromyographic analysis of joint-dependent global synkinesis in the upper limb of healthy adults: Laterality of intensity and symmetry of spatial representation

The intensity and spatial representation of electromyographical (EMG) activity were examined to characterize the effects of limb dominance and movement direction upon global synkinesis (GS). Twenty-two healthy young subjects (11 men, 11 women) with a mean age of 24.7 years participated in this study. Three trials of EMG activities from eight primary muscles in the unexercised limb were recorded when a maximal isometric contraction in various directions was performed by the shoulder, elbow, and wrist of the dominant and non-dominant upper limbs. The features of GS, including intensity and spatial representation, were quantified with standardized net excitation levels (SNE) and relative excitation (RE), respectively. Our data indicated that (1) GS intensity was strongly limb-dependent with a larger SNE level arising when target joints of the non-dominant upper limb were active, (2) the GS intensity was more influenced by movement direction of the non-dominant limb than by that of the dominant limb, (3) the gradient change in GS intensity was observed bilaterally with a larger SNE level associated with contralateral movements of a proximal joint than a distal joint, and (4) GS spatial representations of the upper limbs were patterned and symmetrical, but seemly insensitive to movement direction. Laterality in GS intensity and structured GS spatial representation with symmetry could be a consequence of use-dependent hemispheric organization.

Hemodynamic and metabolic responses to neuronal inhibition

Functional magnetic resonance imaging (fMRI) was used to investigate the changes in blood oxygenation level dependent (BOLD) signal, cerebral blood flow (CBF) and cerebral metabolic rate of oxygen consumption (CMR(O(2))) accompanying neuronal inhibition. Eight healthy volunteers performed a periodic right-hand pinch grip every second using 5% of their maximum voluntary contraction (MVC), a paradigm previously shown to produce robust ipsilateral neuronal inhibition. To simultaneously quantify CBF and BOLD signals, an interleaved multislice pulsed arterial spin labeling (PASL) and T(2)*-weighted gradient echo sequence was employed. The CMR(O(2)) was calculated using the deoxyhemoglobin dilution model, calibrated by data measured during graded hypercapnia. In all subjects, BOLD, CBF and CMR(O(2)) signals increased in the contralateral and decreased in the ipsilateral primary motor (M1) cortex. The relative changes in CMR(O(2)) and CBF were linearly related, with a slope of approximately 0.4. The coupling ratio thus established for both positive and negative CMR(O(2)) and CBF changes is in close agreement with the ones observed by earlier studies investigating M1 perfusion and oxygen consumption increases. These findings characterize the hemodynamic and metabolic downregulation accompanying neuronal inhibition and thereby establish the sustained negative BOLD response as a marker of neuronal deactivation.

Cortico-muscular coupling in a human subject with mirror movements--a magnetoencephalographic study

We studied cortico-muscular coupling in a 15-year-old male suffering from congenital mirror movements (MMs) of hands. Cortex-muscle coherence was analyzed between magnetoencephalographic signals and the electromyograms (EMGs) recorded from both hands and feet during uni- and bilateral isometric contractions. Regardless of the side of the intended contraction, the motor cortex contralateral to the contraction was coupled to the muscles of both hands at 20-25 Hz. No coupling was found from the other, ipsilateral hemisphere. EMGs of the two hands were coupled during both intended uni- and bilateral contractions, but only during unilateral contractions could the coupling solely be explained by cortical activation. We suggest that our subject's MMs result from activation of an ipsilateral corticospinal projection, with involvement of additional synchronizing mechanisms at the subcortical, brainstem, or spinal level during bilateral contraction.

Investigations into the association between cervicomedullary neuroschisis and mirror movements in patients with Klippel-Feil syndrome

BACKGROUND AND PURPOSE

Our purpose was to investigate the association between cervicomedullary neuroschisis and mirror movements in patients with Klippel-Feil syndrome (KFS).

METHODS

We conducted a retrospective analysis of 23 patients with KFS who were seen at our institution during a 10-year period. Sixteen of the 23 patients had undergone adequate axial view cross-sectional imaging of the upper cervical spine. The degree of neuroschisis was assessed for each patient, using an objective scoring system. Twelve patients were evaluated for the presence or absence of mirror movements.

RESULTS

A high percentage of female patients with KFS was noted (17 [74%] of 23 patients). Adequate cross-sectional images were available for 16 of the 23 patients, six (38%) of whom had some form of cervicomedullary neuroschisis. Five of the six patients had been clinically evaluated, and all were shown to have mirror movements. One patient with Chiari II malformation, which obscured evaluation for neuroschisis, also had mirror movements. Of the remaining nine patients without cervicomedullary neuroschisis, six were evaluated, and none of the six had mirror movements. A review of the theoretical neuroanatomic basis of mirror movements is presented herein, and neurosurgical management concerns for patients with KFS are discussed.

CONCLUSION

A strong association exists between cervicomedullary neuroschisis and mirror movements in cases of KFS. Screening of patients with mirror movements may help identify clinically unsuspected KFS and may also help stratify risk within this patient population, identifying patients who might benefit from early neurosurgical intervention.

Motor control in simple bimanual movements: a transcranial magnetic stimulation and reaction time study

OBJECTIVE

Simple reaction time (RT) can be influenced by transcranial magnetic stimulation (TMS) to the motor cortex. Since TMS differentially affects RT of ipsilateral and contralateral muscles a combined RT and TMS investigation sheds light on cortical motor control of bimanual movements.

METHODS

Ten normal subjects and one subject with congenital mirror movements (MM) were investigated with a RT paradigm in which they had to move one or both hands in response to a visual go-signal. Suprathreshold TMS was applied to the motor cortex ipsilateral or contralateral to the moving hand at various interstimulus intervals (ISIs) after presentation of the go-signal. EMG recordings from the thenar muscles of both hands were used to determine the RT.

RESULTS

TMS applied to the ipsilateral motor cortex shortened RT when TMS was delivered simultaneously with the go-signal. With increasing ISI between TMS and go-signal the RT was progressively delayed. This delay was more pronounced if TMS was applied contralateral to the moving hand. When normal subjects performed bimanual movements the TMS-induced changes in RT were essentially the same as if they had used the hand in an unimanual task. In the subject with MM, TMS given at the time of the go-signal facilitated both the voluntary and the MM. With increasing ISI, however, RT for voluntary movements and MM increased in parallel.

CONCLUSIONS

Ipsilateral TMS affects the timing of hand movements to the same extent regardless of whether the hand is engaged in an unimanual or a bimanual movement. It can be concluded, therefore, that in normal subjects simple bimanual movements are controlled by each motor cortex independently. The results obtained in the subject with MM are consistent with the hypothesis that mirror movements originate from uncrossed corticospinal fibres. The alternative hypothesis that a deficit in transcallosal inhibition leads to MM in the contralateral motor cortex is not compatible with the presented data, because TMS applied to the motor cortex ipsilateral to a voluntary moved hand affected voluntary movements and MM to the same extent.

Patterns of motor control reorganization in a patient with mirror movements

OBJECTIVE

To explore motor control reorganization in a 40-year-old, left-handed patient with perinatally acquired mirror movements.

METHODS

We performed simultaneous bilateral recordings of motor evoked potentials (MEPs) following focal transcranial magnetic stimulation (fTMS) and of central silent period (cSP) during unilateral voluntary contraction in abductor pollicis brevis (APB) and abductor digiti minimi (ADM) muscles.

RESULTS

For both muscles the MEP study showed bilateral fast-conducting corticospinal projections from the right undamaged hemisphere, and residual contralateral projections from the left hemisphere. The cSP findings differed in the two muscles: the mirror phenomenon was bilateral in the ADM, but present only on the right side in the APB muscles; the mirror activity of right ADM and APB muscles was inhibited only by fTMS of the ipsilateral right motor cortex; the mirror phenomenon in the left ADM muscle was inhibited only by fTMS of the contralateral right motor cortex.

CONCLUSIONS

Mirror movements of right APB and ADM muscles were sustained by the ipsilateral connections from the undamaged motor cortex, while the mirror phenomenon in the left ADM muscle could be explained by hypothesizing a bilateral activation of motor cortices. This previously unreported electrophysiological picture demonstrates that different patterns of motor control may realize after perinatal cerebral lesions, even in different distal muscles of the same patient.

Dissociation of the pathways mediating ipsilateral and contralateral motor-evoked potentials in human hand and arm muscles

1. Growing evidence points toward involvement of the human motor cortex in the control of the ipsilateral hand. We used focal transcranial magnetic stimulation (TMS) to examine the pathways of these ipsilateral motor effects. 2. Ipsilateral motor-evoked potentials (MEPs) were obtained in hand and arm muscles of all 10 healthy adult subjects tested. They occurred in the finger and wrist extensors and the biceps, but no response or inhibitory responses were observed in the opponens pollicis, finger and wrist flexors and the triceps. 3. The production of ipsilateral MEPs required contraction of the target muscle. The threshold TMS intensity for ipsilateral MEPs was on average 1.8 times higher, and the onset was 5.7 ms later (in the wrist extensor muscles) compared with size-matched contralateral MEPs. 4. The corticofugal pathways of ipsilateral and contralateral MEPs could be dissociated through differences in cortical map location and preferred stimulating current direction. 5. Both ipsi- and contralateral MEPs in the wrist extensors increased with lateral head rotation toward, and decreased with head rotation away from, the side of the TMS, suggesting a privileged input of the asymmetrical tonic neck reflex to the pathway of the ipsilateral MEP. 6. Large ipsilateral MEPs were obtained in a patient with complete agenesis of the corpus callosum. 7. The dissociation of the pathways for ipsilateral and contralateral MEPs indicates that corticofugal motor fibres other than the fast-conducting crossed corticomotoneuronal system can be activated by TMS. Our data suggest an ipsilateral oligosynaptic pathway, such as a corticoreticulospinal or a corticopropriospinal projection as the route for the ipsilateral MEP. Other pathways, such as branching of corticomotoneuronal axons, a transcallosal projection or a slow-conducting monosynaptic ipsilateral pathway are very unlikely or can be excluded.

Differences between 'congenital mirror movements' and 'associated movements' in normal children: a neurophysiological case study

In this study we analysed how far physiological associated movements in normal children (which may be present up to the age of 10 years) share the same physiological mechanism with clinically apparent mirror movements. Transcranial magnetic stimulation (TMS) and kinematic movement analysis were applied in a 4-year-old child with congenital mirror movements (CMM). The results were compared with a normative data base of clinically normal children. In the child with CMM focal TMS of one motor cortex induced bilaterally symmetrical responses in distal and proximal upper extremities muscles with identical ipsi- and contralateral latencies. Also kinematic analysis showed a precise symmetrical onset of intended and unintended contralateral movements, whereas normal children with associated movements showed a variable movement onset delay between extremities. The data suggest a different physiological mechanism underlying these two varieties of elementary associated motor activity in childhood.

Quantitative assessment of mirror movements after stroke

BACKGROUND AND PURPOSE

Mirror movements (MM) are involuntary synchronous movements of one limb during voluntary unilateral movements of the opposite limb. We measured MM in stroke and control subjects and evaluated whether MM after stroke are related to motor function.

METHODS

Twenty-three patients and 16 control subjects were studied. A computerized dynamometer was used during two squeezing tasks to measure intended movements from the active hand as well as MM from the opposite hand. Motor deficits were measured with the arm motor component of the Fugl-Meyer scale.

RESULTS

During paretic hand squeezing, MM in the unaffected hand were detected in 70% (repetitive squeeze) to 78% (sustained squeeze) of stroke patients. For both tasks, this was significantly (P < 0.05) greater than the incidence of MM in the paretic hand or in either hand of control subjects (17% to 44%), except when compared with the incidence of MM in the dominant hand of control subjects (56%; P = 0.17). The incidence of MM in the paretic hand was not significantly different from that seen in either hand of control subjects. Patients with MM in the unaffected hand had significantly greater motor deficit than patients without MM. Patients with MM in the paretic hand had significantly better motor function than patients without MM.

CONCLUSIONS

Simultaneously recording motor performances of both hands provides precise information to characterize MM. MM in the unaffected hand and in the paretic hand are associated with different degrees of motor deficit after stroke. Evaluation of MM may be useful for studying mechanisms of stroke recovery.

Ontogeny of ipsilateral corticospinal projections: a developmental study with transcranial magnetic stimulation

Transcranial magnetic stimulation (TMS) has been used to describe the maturation of the corticospinal tract in children. Ipsilateral corticospinal connections have been demonstrated with TMS in patients with congenital mirror movements, in patients after hemispherectomy, and in children with hemiplegic cerebral palsy. The goal of the study was to find out whether corticospinal ipsilateral projections in children can be demonstrated during the first decade of life as part of normal ontogeny. For this purpose, we examined 50 normal children (age range, 3-11 years) with focal TMS over the left and right hemispheres to target muscles in proximal and distal parts of the upper extremity (first dorsal interosseus, biceps brachii, and brachioradialis). To lower the stimulation threshold, we stimulated under voluntary preinnervation. In two-thirds of the children we elicited ipsilateral motor evoked potentials (MEPs). This occurred more often in proximal than in distal muscles. The latency of the ipsilateral MEPs was about 12 to 14 msec longer than the usual contralateral response. From the age of 10, and in adults, ipsilateral MEPs could not be detected. Also considering lesion data from adult patients, the most likely explanation for the disappearance of ipsilateral corticospinal connections after the age of 10 years is an increasing transcallosal inhibitory influence during development. The presence of ipsilateral corticospinal connections appears to be a normal state in ontogeny.

The effects of external load on movement-related changes of the sensorimotor EEG rhythms

The effects of external load opposing brisk voluntary extension of the right index finger on the EEG rhythms in the left and right sensorimotor hand area were studied in 13 right-handed subjects. Four levels of external loads corresponding to the weights of 0 g (no load), 30 g, 80 g and 130 g were used. The effects of external load on EEG rhythms were the following: (i) prior to movement, the desynchronisation of beta-rhythms (18-25 Hz) over the contralateral sensorimotor area was greater under the two largest loads as compared to the 0 g load. However, beta-desynchronisation at ipsilateral electrodes was larger under the 80 g load than under the 130 g load, presumably due to a transcallosally mediated inhibition exerted by the highly excited contralateral motor area; (ii) the mu-rhythm desynchronisation continued over both hemispheres for about 0.3-0.4 s after movement and the largest load was accompanied by the longest mu-rhythm desynchronisation; (iii) the post-movement beta-synchronisation was also longer under the heaviest load (130 g) as compared to the no-load condition (0 g), especially in subjects who prolonged their total movement time under the heaviest load. The results show that (i) the movement-related desynchronisation and synchronisation of sensorimotor EEG rhythms is influenced by external load opposing finger movement, and (ii) the effects of external load differ for the mu- and beta-rhythms.

Role of the ipsilateral motor cortex in mirror movements

The mechanism of mirror movements in two patients was investigated; one with congenital mirror movement, the other with schizencephaly. Transcranial magnetic stimulation on one side elicited motor evoked potentials (MEPs) in their thenar muscles on both sides with almost the same latencies, minimal thresholds, and cortical topographies. During voluntary contraction of the thenar muscle on one side, contralateral transcranial magnetic stimulation induced a silent period not only on the voluntary contraction side but on the mirror movement side and of the same duration. By contrast, ipsilateral transcranial magnetic stimulation elicited MEPs without silent periods in both muscles. With intended unilateral finger movements, an H2(15)O-PET activation study showed that the regional cerebral blood flow increased predominantly in the contralateral sensorimotor cortex, as seen in normal subjects, although mirror movements occurred. It is considered that the ipsilateral motor cortex plays a major part in the generation of mirror movements, which may be induced through the ipsilateral uncrossed corticospinal tract.

Stretch reflexes of the proximal arm in a patient with mirror movements: absence of bilateral long-latency components

The stretch reflex responses evoked by unilateral limb displacement in distal (first dorsal interosseus (FDI)) and in proximal (biceps brachii (Bb)) arm muscles were studied during matched bilateral contractions in a patient with congenital mirror movements. In this patient unilateral transcortical magnetic stimulation (TMS) elicited not only the normal contralateral EMG response but also a clear ipsilateral component in the EMG of both proximal and distal arm muscles. As expected from previous studies, the ipsilateral FDI muscle responded to stretch of the index finger with short- (M1) and long-latency (M2) reflex components. In addition, the FDI contralateral to displacement exhibited an abnormal mirrored response corresponding to the M2 interval. In contrast, whereas the ipsilateral Bb responded to imposed elbow extension with a marked M1/M2 reflex response, no mirroring of either reflex component was apparent in the contralateral Bb EMG. If the mirroring of the M2 in the FDI is accepted as evidence for the transcortical nature of the M2 reflex response, then it follows that the absence of such mirroring in the Bb indicates that a transcortical mechanism cannot play a major role in the generation of long-latency stretch reflex responses in proximal arm muscles.

Movement-related cortical potentials in persistent mirror movements

Mirror movements (MMs) are involuntary movements executed on one side of the body during voluntary movements of the contralateral homologous body parts which may abnormally persist into adulthood. In 6 subjects affected by persistent MM with autosomal dominant inheritance, movement-related cortical potentials (MRCPs) during self-paced, voluntary extensions of either the left or right middle finger were recorded from 30 EEG electrodes simultaneously with the electromyogram (EMG) of both extensor digitorum communis muscles. The negative potentials before and during EMG onset were evaluated statistically for the two electrodes next to the cortical hand areas. A comparison with 7 normal subjects revealed no marked differences for the Bereitschaftspotential (BP) and the negative slope (NS'). Only in the periods around EMG onset (from -50 to +50 msec) a significant difference between both groups was found. The MM subjects showed fairly symmetric potentials over the right and left hemispheres, whereas the potentials of the control subjects were lateralized to the hemisphere contralateral to the intended movement. No difference was found for the amplitude of the maximum negative peak of MRCP following EMG onset. Our data showed no evidence for a different type of movement preparation in MM subjects as compared to normals. We propose that the additional ipsilateral cortical activation around movement onset may be the cortical mechanism, which compensates for abnormal ipsilateral corticospinal pathways in subjects with persistent MM.

Hemispheric asymmetry of transcallosal inhibition in man

The transcallosal connecting fibres linking corresponding projection areas of the same muscles of the right and left primary motor cortex may play an important role in control of unilateral movements. It appears that they have mainly inhibitory effects. This was further evaluated by transcranial magnetic stimulation using two focal coils placed on the optimal positions, i.e. the positions with the lowest thresholds at the motor representation areas of the first dorsal interosseous muscle of the left and right sides. A conditioning stimulus was given to one hemisphere 10 ms prior to the test stimulus at the opposite hemisphere. The inhibition was evaluated as relative amplitude reduction. Eleven normal right-handed subjects and 11 normal left-handed subjects participated in this study. Handedness was evaluated by the Oldfield inventory. It was found that in right-handers the inhibition after stimulation of the "dominant" left hemisphere was more marked than after stimulation of the "non-dominant" right hemisphere. In contrast, the group of left-handed subjects showed inhomogeneous findings with either right- or left-side predominant inhibition. It is concluded that not handedness but hemispheric dominance contributes to the laterality of inhibition. The results point to a superior role of the language-dominant hemisphere in governing inter-hemispheric control of motor cortical connections, supporting the view that the "language-dominant" hemisphere is also "motor dominant".

Persistent mirror movements: force and timing of "mirroring" are task-dependent

A simple isometric motor task was used to quantify intended and unintended finger movements in two subjects (father and son) with persistent mirror movements. One hand voluntarily changed grip force between thumb and index finger at different amplitudes and frequencies, while the other hand was to maintain a constant force. During all experimental conditions the "steady" hand showed insuppressible, highly cross-correlated contractions, compatible with bilateral distribution of a single motor command to the spinal cord. However, these associated movements were not strictly mirror images, nor did they show a fixed relationship to the voluntary movements across experimental conditions. The ratio of mirror to voluntary movement ranged from 1.4 to 19.1% and from 3.4 to 78.4% in the two subjects and was directly related to voluntary strength and speed. At maximum speed, mirror activity tended to precede voluntary activity, while it was delayed in slow force changes. Comparable time lags were not found in control subjects instructed to simulate mirror movements. We conclude that neuronal mechanisms in addition to bilateral corticomotoneuronal connections are at work in persistent mirror movements.

Abnormal projection of corticospinal tracts in a patient with congenital mirror movements

A 31 year-old woman with familial congenital mirror movements not associated with other neurological defects underwent a detailed neurophysiological evaluation including: voluntary electromyographic activity recorded from upper limbs in response to acoustic stimuli, motor evoked potentials from the thenar muscles to focal transcranial magnetic stimulation, F waves from upper extremities, scalp somatosensory evoked potentials and long-latency responses from thenar muscles to electric stimulation of the median nerve. The results were consistent with the presence of fast-conducting pathways connecting each hand motor cortex with both contra- and ipsilateral spinal motoneurones.

Persistent mirror movements: a clinical study of 17 children, adolescents and young adults

The author reports a series of 17 cases of congenital mirror movements of the hands and forearms (and in a few cases the toes), but no other signs of gross neurological deviation. This is an uncommon disorder that often goes unrecognised. This slight but definite disability seems to persist largely unchanged at least into the late teens, and probably into adulthood. A hereditary background is suggested in about half of the cases. Various neuropsychiatric deficits were seen in combination with the mirror movements in about half the cases, as well as a high incidence of non-right-handedness. Although the clinical heterogeneity of the disorder was clear, the phenomenon of mirror movements was similar in all cases. Recent progress in understanding the pathophysiology of this disorder is reviewed.

Cortically evoked motor responses in patients with Xp22.3-linked Kallmann's syndrome and in female gene carriers

Patients with Kallmann's syndrome show hypothalamic hypogonadism, hyposmia, and congenital mirror movements. As a correlate, a defect of gonadotropic neuron migration into the brain was recently detected. Considering abnormal outgrowth of neurons also as a possible substrate underlying mirror movements, we studied 3 patients and 2 asymptomatic female gene carriers from a kindred with proven linkage to Xp22.3, using focal transcranial magnetic stimulation of motor cortex hand areas with a figure-eight coil. In all 3 affected brothers, bilateral responses could be evoked almost simultaneously in their thenar muscles (slight latency differences were statistically insignificant). In contrast, the mother and the maternal aunt showed only unilateral, normal thenar responses, even with maximum tolerable stimulator output and high signal amplification. Correspondingly, mirror movements were present in the patients, but not in the gene carriers. Bilaterality of cortically evoked hand muscle responses and mirror movements, therefore, behaved as X-chromosomal recessive traits. A likely cause might be a disorder of neuronal outgrowth in the motor system, particularly of inhibitory callosal fibers. For normal anatomical development of the motor system, one intact Xp22.3 gene seems necessary.